The invention relates to a pneumatic rubber tire with an outer, circumferential tread wherein said tread has a tread configuration comprised of spaced apart, raised lugs designed to be shock absorbingly ground contacting, wherein said tread and lugs are comprised of rubber selected from butyl rubber, halobutyl rubber or isobutylene-derived rubber. Said tire may also contain sidewalls comprised of one or more of such rubber. Thus, such tread is a combination of specific structural configuration and specific rubber composition to create a shock absorbing effect.
Large pneumatic tires are used for various agricultural vehicles, such as for example tractors. Such tires may have a diameter, for example, of at least 58 inches (at least about 147 cm).
Such tire treads generally contain significantly raised lugs which are designed to be ground engaging.
It can readily be seen that such tractor tires, with lugs designed to be ground engaging, present only small portion of the tread to the ground, at least insofar as the surface of the lugs themselves are concerned as compared to typical passenger tire treads.
Accordingly, such tractor tires rely more on the weight of the associated tractor to provide tire tread traction over the ground than passenger tires.
Also, it can be readily visualized that shocks originating by the tire traveling over irregularities of the ground are readily transmitted to the wheel, and hence to the axle, of the associated vehicle and thereby to transmit the associated shock to the vehicle which, in turn, can result in a discomfort to the individual driving the vehicle.
In such circumstance, such agricultural vehicles typically rely upon the shock absorbency of the raised lug configuration of the tire tread to provide a degree of shock absorbency for the comfort of vehicle operator.
Accordingly, it remains desirable, in many circumstances, to increase the shock absorbency for such vehicles having a tread of such spaced apart, raised lug configuration.
While butyl rubber has heretofore been suggested for use in various tire treads, it is considered that such adaptation by this invention is a significant departure from past practice. In particular, this invention requires a combination of tread configuration with spaced apart significantly raised lugs designed to be ground engaging in combination with a shock absorbing rubber composition. Thus such lugs are designed to be shock absorbingly ground contacting.
In the description of this invention, terms such as xe2x80x9ccompounded rubberxe2x80x9d, xe2x80x9crubber compoundxe2x80x9d and xe2x80x9ccompoundxe2x80x9d, if used herein, refer to rubber compositions composed of one or more elastomers blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms xe2x80x9celastomerxe2x80x9d and xe2x80x9crubberxe2x80x9d might be used herein interchangeably. It is believed that all of such terms are well known to those having skill in such art.
A reference to glass transition temperature, or Tg, of an elastomer or elastomer composition, where referred to herein, represents the glass transition temperature(s) of the respective elastomer or elastomer composition in its uncured state or possibly a cured state in a case of an elastomer composition. A Tg can be suitably determined by a differential scanning calorimeter (DSC) at a temperature rate of increase of 10xc2x0 C. per minute.
Disclosure and Practice of the Invention
In accordance with this invention, a pneumatic rubber tire is provided, particularly a farm tractor tire having a circumferential rubber tread configured with spaced apart raised lugs designed to be shock absorbingly ground contacting, a supporting carcass underlying said tread, a pair of spaced apart beads, and rubber sidewalls extending radially outward from said beads to the peripheral edges of said tread, wherein said raised lugs have a height from their surface intended to be ground contacting to the base thereof at the surface of said tread in a range of about 12.5 cm to about 80 cm, wherein the tread rubber composition of said tread and lugs is comprised of, based upon parts by weight per 100 parts by weight rubber (phr):
(A) about 70 to about 100, alternately about 75 to about 90, phr of at least one isobutylene-based rubber selected from
(1) butyl rubber as a copolymer of isobutylene and isoprene wherein said copolymer contains from about 2 to about 6 weight percent units derived from isoprene,
(2) halobutyl rubber as a halogenated butyl rubber where the halogen is selected from bromine or chlorine, preferably bromine, or
(3) brominated copolymer of isobutylene and paramethylstyrene, and
(B) zero to about 30, alternately about 10 to about 25, phr of at least one diene-based elastomer selected from polymers of isoprene and/or 1,3-butadiene and copolymers of styrene with isoprene and/or 1,3-butadiene.
In an additional aspect of the invention, outer rubber sidewalls individually extending radially from the peripheral edge of said tread to spaced apart beads of the tire carcass which to the said tire are comprised of said isobutylene-based rubber composition which may be the same or different from the specific rubber composition used for said tire tread.
In a further aspect of the invention, a thin layer of a rubber composition as vulcanizable blend of diene-based rubber and one or more of said isobutylene-based rubbers, in a weight ratio of from 30/70 to 70/30 of such diene-based to said isobutylene-based rubbers, may be positioned between said tread and said carcass.
A significant aspect of the invention is the utilization of the prescribed shock absorbing rubbers for the significantly raised tread lugs as compared to diene-based rubbers of significantly reduced shock absorbing ability, particularly where the substantial tread lug rubber composition is required to have a rebound value (23xc2x0 C.) of less than 25 and a Shore A hardness (23xc2x0 C.) of greater than 60.
A further significant aspect of the invention is the utilization of said isobutylene-based rubber composition for said tire sidewalls, also with said rebound and hardness value characterizations, to extend the shock dampening effect from the tread to the tire carcass beads.
In particular, the so-called shock absorbing ability of the prescribed rubbers is largely dependent upon a damping effect of such rubbers for shocks experienced by such raised tread lugs and, in the practice of this invention, it is preferred and is intended to be required, that the tread rubber composition, particularly for the shock-absorbing tread lugs, has a significant dampening effect as exhibited by having a rebound physical property value (ASTM D 1054 at 23xc2x0 C. of less than 25 and, further, also particularly for the shock-absorbing tread lugs, a sufficient stiffness to enhance the dimensional stability of the tread lugs as exhibited by a Shore A hardness value (ASTM D2240 at 23xc2x0 C.) of greater than 60.
For such raised lug configured tread, said optional minor amount of a diene based rubber may be used to enhance cured adhesion of the tread rubber to a diene-based rubber composition of the tire carcass (or tread base) which supports the circumferential tread.
Such butyl rubber may be prepared, for example, by copolymerizing isobutylene with a minor amount of isoprene. The halobutyl rubber can be prepared by halogenating such butyl rubber, for example by brominating butyl rubber.
The brominated isobutylene/paramethylstyrene copolymer rubber typically has repeat units derived from polymerization of monomers comprising at least isobutylene and paramethylstyrene. The bromine content may be from about 0.1 to about 4 percent by weight and more desirably from about 0.1 to about 2.5 percent by weight, based upon the weight of the brominated polymer. The isobutylene content is desirably from about 85 to about 99.4, or up to about 99.8, alternately about 88 to about 97.9, weight percent. The paramethylstyrene content is usually from about 1 to about 14, alternately about 2 to about 11, weight percent. Repeat units of other monomers may also be present or may be excluded. Many such polymers are commercially available from Exxon Chemical company as Exxpro(trademark) and a preparation of such polymers is exemplified, for example, in U.S. Pat. No. 5,162,445 which is hereby incorporated by reference in its entirety.